...was proposed by Bill Napier in a paper last year. Here's the abstract...

Title:
A mechanism for interstellar panspermia

Authors:
Napier, W. M.

Affiliation:
Armagh Observatory, College Hill, Armagh BT61 9DG;

Journal:
Monthly Notices of the Royal Astronomical Society, Volume 348, Issue 1, pp. 46-51.

Publication Date:
02/2004

Abstract
Metre-sized boulders ejected from the Earth by large impacts are destroyed through collisions and erosion by impacting zodiacal cloud dust particles. The time-scale for such disintegration in a dense zodiacal cloud may be <~ 104 yr. Once reduced to a critical size, the particles are rapidly ejected from the Solar system by radiation pressure. The critical size for ejection is of the order of a micron, large enough to protect groups of micro-organisms within them from solar ultraviolet irradiation. Such life-bearing particles are ejected at a mean rate of ~ 1020 per million years. During passages of the Solar system through or close to dense molecular clouds, a significant proportion of the particles may be incorporated into protoplanetary systems and protected from cosmic rays within growing planetesimals. The specific number density of micro-organisms so deposited is highest in small, dense molecular clouds. On the assumption that this ejection mechanism is common in other planetary systems environmentally capable of supporting life, a `chain reaction' may seed the disc of the Galaxy within a few billion years. In that case it is unlikely that life originated on Earth.

...the chief difference seems to be that Adams and co. assume the ejected rocks remain whole during the process, but as Napier points out this is highly unlikely. I think it's reasonable to assume that Life, once begun, has quite effectively contaminated the rest of the Galaxy. Question is whether other kinds of life have arisen and have successfully out-competed our kind of life elsewhere.

Adam